1. Field of the Invention
The present invention relates to a contact lens having improved properties. More particularly, the invention relates to an ionic contact lens having improved dimensional stability and water retention. The contact lens is made of an ionic polymer. The invention also relates to an ionic polymer useful in contact lenses, and also to methods of making and using the polymer and contact lens.
2. Description of Related Art
As described in U.S. Pat. No. 5,532,289 and U.S. Ser. No. 08/674,275 filed Jul. 1, 1996, both incorporated herein by reference in their entirety, the ability of a hydrogel lens to maintain its water saturated state is important for maximum lens stability. Hydrogel lenses dehydrate because water evaporates continuously from their surface. Dehydration of a contact lens results in a change in the dimensions of the lens, hence dehydration has a direct effect upon dimensional stability. Conventional contact lenses undergo a significant degree of dehydration during use and, accordingly, have a significant degree of dimensional instability, particularly at higher water contents.
Further, rehydration is important to the dimensional stability of a contact lens. If a lens material absorbs water rapidly, then the lens will more closely return to a water-saturated state during each blink, when the lens is bathed in tear fluid. Therefore, as a lens begins to dehydrate, a characteristic of rapid rehydration is extremely advantageous for maintaining saturation and maximum stability of the lens.
Unfortunately, conventional ionic contact lens development either has ignored the effect of rehydration rate upon lenses or has constructed lenses of materials with a less than optimal rate of rehydration. As such, there remains a need for an ionic contact lens possessing superior dimensional stability and having a low rate of dehydration and a high rate of rehydration.
Ionic hydrophilic lenses introduced in recent years have been based on either existing materials employing new production technology or slight modifications of known compositions. This lack of progress in the ionic soft lens field has resulted in a large variety of lens designs, but a narrow choice of lens materials and a narrow range of lens stabilities as measured by water balance ratio.
Limited choice in lens material is problematic when attempting to fit contact lenses on patients subject to a wide variety of physiological and environmental conditions. For example, an array of factors affect contact lens comfort and stability, such as, tear quantity, ambient humidity, prolonged open eye periods, and airflow around the eye. Especially difficult cases are posed by patients with dry eyes.
The dehydration of hydrophilic lenses is a major problem, affecting lens movement, lens power, oxygen permeability, and comfort. Various factors including patient physiology, environment, lens design, and lens material significantly influence the rate of dehydration, as described in Andrasko, Hydration Levels and Oxygen Transmissivities of Ophthalmic Polymer In Situ, Theses, Ohio State University, 1980, and McCarey et al. pH, Osmolarity and Temperature Effects on the Water Content of Hydrogel Contact Lenses, Contact and Intraocular Lens Medical Journal 8, 158-167, 1982. Thicker lenses also appear to dehydrate less than thinner lenses, as described in Businger et al., Die Beeinflussung der Dehydration von hydrophilen Kontaktlinsen durch verschiedene Linsenparameter, Deutsche Optiker Zeitung 40, 99-102 (1985).
While a variety of hydrophilic lens materials are available, they differ only slightly in their rates of dehydration, as described in Helton et al., Hydrogel Contact Lens Dehydration on Rates Determined by Thermogravimetric Analysis CLAO 17, 59-61 (1991). These factors are particularly pronounced during the cold season or in dry environments, see Andrasko et al., The Effect of Humidity on the Dehydration of Soft Contact Lenses on the Eye, Int. Cl. Clinic 7, 30 (1982) and Eng et al., The Wearing of Hydrophilic Contact Lenses Aboard a Commercial Jet Aircraft: 1 Humidity Effects on Fit, Aviat. Space Environ. Med. 53,235 (1982).
Non-ionic materials containing glyceryl methacrylate have been reported to have improved internal water retention over poly-HEMA, see Pescosolido, et al., Nuclear Magnetic Resonance Study of Dehydration in Glyceryl-methyl-methacrylate Contact Lens, Contactologia 15D, 64 (1993), see Businger, GMA/HEMA: First Report on a Clinical Trial, Contact Lens Spectrum (August 1995), see Benz and Ors, New Materials Demand More Accurate Measurements of Performance Contact Lens Spectrum (July 1997).
The ability of a hydrogel lens to maintain its saturated state is important for lens stability. All hydrogel lenses dehydrate. Water evaporates from the surface of a hydrogel lens continuously. The amount of water loss that a lens will experience depends upon the dehydration/rehydration behavior of the particular lens material, the quantity of tears deposited on the lens with each blink, the ambient humidity, temperature and air flow around the eye.
Superior dehydration/rehydration behavior of soft lens materials provides the material with increased dimensional stability. If a soft lens material can be made to dehydrate (allow evaporation) more slowly, then the lens will remain closer to its saturated state. Equally important is the importance of rehydration. If a lens material can be made to re-absorb water more rapidly, then the lens can return to a state closer to saturation during each blink, when the lens is bathed in tear fluid. Thus, an ideal soft contact lens is one constructed from a composition that is both slow to dehydrate and quick to rehydrate.